A cryogenic air separation plant is an apparatus in which feed air is liquefied and then separated by distillation into nitrogen, oxygen, and so on. When the distillation is performed, a process of removing impurities such as moisture and carbon dioxide which are frozen at low temperature so as to obstruct a pipe, so-called pretreatment, is performed in a feed air purifier. As this pretreatment, a temperature swing adsorption method (TSA method) is generally used, which uses two or more adsorption columns placed in parallel. A moisture adsorbent such as activated alumina, silica gel, or zeolite is filled upstream of the adsorption column to which feed air flows, and a carbon dioxide adsorbent such as an Na-X type zeolite is filled downstream. A temperature swing adsorption method alternately performs an adsorption process in which impurities such as moisture and carbon dioxide are removed by adsorption at low temperature and a regeneration process in which adsorbents are regenerated at high temperature.
Hereinafter, an example of operation during steady operation of a feed air purifier (hereinafter, referred to as TSA apparatus) by using a temperature swing adsorption method is explained with reference to FIG. 1. In this example, an adsorption column 5a is assumed to perform a regeneration process, and an adsorption column 5b is assumed to perform an adsorption process. FIG. 1 is a configurational illustration representing an example of a pretreatment part for feed air in a cryogenic air separation plant. First, feed air which is introduced from the atmosphere is compressed so as to reach a predetermined pressure (400 to 1,000 kPa (hereinafter, every pressure in the present specification represents an absolute pressure.)) by a feed air compressor 1, and then is cooled (at 5° C. to 45° C.) by a cooling apparatus 2. At this time, condensed water is ejected by a drain separator 3. Next, the condensed feed air with saturated moisture at a cooling temperature flows in the adsorption column 5b through a valve 4b, and the impurities such as the moisture and the carbon dioxide in the feed air are adsorbed by the adsorbent in the adsorption column 5b. Subsequently, purified feed air flows in an air separation section 8 through a line 7 and valves 6b, 18.
In the adsorption column 5b performing the adsorption process, a mass transfer zone of adsorbed components in the adsorbent layer proceeds from upstream of the adsorption column to which feed air flows to downstream. Therefore, the adsorption process is finished before the concentrations of the impurities in the purified air reach a limitation value, which is problematic in the air separation section 8.
After the end of the adsorption process, a regeneration process is started. The regeneration process includes four steps of a depressurizing step, a heating step, a cooling step, and a pressurizing step. In the depressurizing step, the valves 4b, 6b are closed, and the atmosphere-releasing valve 9b is opened. As a result, the gas held in the adsorption column 5b is ejected to the atmosphere through a silencer 10, and the pressure in the adsorption column 5b is decreased to atmospheric pressure.
In the following heating step, valves 12, 14b are opened. As a result, a part of the exhaust gas from the air separation section 8 flows in a heater 13 through a line 11 as a purge gas. After being heated to 150° C. to 250° C., the purge gas flows in the adsorption column 5b through the valve 14b. The inflow of the heated purge gas heats the adsorbent; therefore, the impurities such as moisture and carbon dioxide adsorbed to the adsorbent are desorbed from the adsorbent and flow out together with the purge gas flow. The purge gas flowing out is ejected to the atmosphere through an atmosphere-releasing valve 9b and the silencer 10.
FIG. 2 is a schematic graph representing an example of the temperature change of the purge gas in the adsorption column 5b performing the regeneration process as a function of the position. As shown in FIG. 2 (a), the zone with a high temperature (heat-zone) occurs in the adsorption column 5b due to the inflow of the heated purge gas. This heat-zone follows the purge gas flow to gradually migrate to the atmosphere-releasing valve 9b. After the end of the heating step, the cooling step is started. In the cooling step, the valve 12 is closed, and the valve 15 is opened. The purge gas does not flow in the heater 13 and directly flows in the adsorption column 5b at a low temperature. This purge gas cools the adsorbent. Also, as shown in FIGS. 2 (b), (c), and (d), the heat-zone is pushed by the low-temperature purge gas flow, migrates to the atmosphere-releasing valve 9b, and then is pushed out of the adsorption column 5b. The impurities are completely ejected from the adsorbent, and the temperature of the adsorbent becomes appropriate for the next adsorption process. Herein, the example represented by FIG. 2 is the same as in the case where the adsorption column 5a performs the regeneration process.
FIG. 3 is a graph representing an example of the temperature change of the purge gas in the adsorption column 5b performing the regeneration process during steady operation as a function of time. A moisture absorbent and a carbon dioxide absorbent are assumed to be deposited in a lower layer and an upper layer, respectively. When the heating step is started, the temperature at the top of a carbon dioxide adsorbent, which is represented by a solid line in FIG. 3, is steeply increased with the inflow of the heated purge gas from the upper part of the adsorption column 5b, and is steeply decreased when the cooling step is started.
The temperature at a border part of the moisture adsorbent and the carbon dioxide adsorbent located downstream of the purge gas flow, which is represented by a dashed line, starts to be smoothly increased after a while from when the heating step is started, keeps a certain temperature, and then starts to be smoothly decreased after a while from when the cooling step is started. The temperature at the outside of the bottom part of the moisture adsorbent (the purge gas-outflowing part) located further downstream (the atmosphere-releasing valve 9b side), which is represented by a bold solid line, starts to be smoothly increased and decreased after a while from when the cooling step is started. Herein, the example represented by FIG. 3 is the same as in the case where the adsorption column 5a performs the regeneration process.
In this way, the flow rate of the purge gas, the heating capacity of the heater, and the allocation of time for a heating step and a cooling step are decided so that the temperature of the moisture adsorbent is increased to a predetermined value during the cooling step and decreased to about the temperature at which feed air is fed before the adsorption process is started.
Subsequently, in a pressurizing step, the valves 14b, 15, and the atmosphere-releasing valve 9b are closed, and a valve 17b is opened. As a result, a part of purified air from the adsorption column 5a performing the adsorption process is returned to the adsorption column 5b through the line 7 and the line 16 and pressurizes the adsorption column 5b to the pressure necessary for the next adsorption process.
At the end of a pressurizing step, the valve 17b is closed, and the valves 4b, 6b are opened again. Then, the adsorption process is started again in the adsorption column 5b. For example, in the case of a two column system, the time for an adsorption process corresponds to the time for a regeneration process from a depressurizing step to the end of a pressurizing step, and the time necessary for each process is 2 to 4 hours. In this case, the adsorption columns 5a, 5b are exchanged alternatively so as to continuously feed purified feed air to the air separation section 8.
Since the cryogenic air separation plant takes a long time to cool the inside of the air separation section 8 from ambient temperature to a cryogenic temperature, the continuous operation is usually performed without stopping frequently. However, a cryogenic air separation plant is urgently stopped for some reasons or stopped according to a plan for a security check, and a TSA apparatus is also urgently stopped for some reasons or stopped according to a plan.
When a TSA apparatus performing the steady operation is stopped and the stop period becomes long, the impurities such as moisture and carbon dioxide are diffused in the adsorption column 5b performing the adsorption process even though the adsorption column 5b is kept in a sealed state. Therefore, there is a case where the impurities pass when an adsorption process is performed from the time point of stopping the TSA apparatus after the restart of the TSA apparatus, and the concentration of the impurities of the purified air can be increased more than during the steady operation and can exceed the limit value.
Meanwhile, in the adsorption column 5a performing the regeneration process, the heat introduced for the regeneration of adsorbent can be released outside due to heat transfer when the TSA apparatus is stopped for a long time. Therefore, when the regeneration process is performed from the time point of stopping the TSA apparatus after the restart of the TSA apparatus, the regeneration of the adsorbent becomes insufficient due to lack of heating, and the concentrations of the impurities in the purified air can be increased more than during the steady operation in the adsorption process after alternation.
In order to solve the aforementioned problems, a self regeneration operation is conventionally performed before feeding a purified air to the air separation section 8 after the restart of the TSA apparatus. This self regeneration operation is the following operation: reducing the flow rate of the feed air, which flows from the feed air compressor 1 to the adsorption column 5b, to less than during the steady operation so as to make a state of a low load; letting the purified air, which flowed out from the adsorption column 5b, flow in the adsorption column 5a while closing the valve 18 between the TSA apparatus and the air separation section 8; and performing an adsorption process and a regeneration process once or more. The changed states in the each adsorption column that occurred during the stop period are cleared by using the self regeneration operation, and then the steady operation is started, thereby preventing the concentrations of the impurities in purified air from increasing.
Also, a method other than the self regeneration operation in the case of not performing the urgent stop but the planned stop is disclosed in Japanese Unexamined Patent Application, First Publication No. 2002-168561. In paragraph 0029 of this publication, the following is described: “the adsorbent in an adsorbing column in a rest condition is regenerated by the nitrogen gas obtained in an air separation section S2, thereby preventing the purification efficiency of an adsorption purification apparatus 12 from decreasing”, and a method of preventing the purification efficiency of thr TSA apparatus from decreasing is described.
However, it is not economical to use nitrogen gas which is a product. Also, the self regeneration operation takes at least 4 hours in the case of a two column-alternating system since the time necessary for an adsorption process or a regeneration process is 2 to 4 hours, and purified gas cannot be fed to a cryogenic air separation plant in this period. Therefore, there is a problem in that the restart of a cryogenic air separation plant is late.
[Patent reference 1]
Japanese Unexamined Patent Application, First Publication No. 2002-168561